1. Field of the Invention
The present invention relates to an electric double layer capacitor.
2. Description of the Background Art
As disclosed in Japanese Patent Application Laid-Open No. 11-150042 (1999), electric double layer capacitors are provided with polarized electrodes (positive electrode and negative electrode) which face each other with a separator placed in between so that the capacitance of the electric double layers, which are formed on the surface of the polarized electrodes in the electrolytic solution, is used. Electric double layer capacitors are characterized in that an extremely large capacitance can be obtained in comparison with general capacitors such as aluminum capacitors, they have started being used in a wide range of applications such as a back up for electronic equipment, power storage for consumer electronics and copying machines, power supply for the start up from an idle stop of an automobile, a power supply for a hybrid automobile, and power storage for peak shaving and leveling of wind power and photovoltaic power generation, and are expected to be key devices that are useful for energy conservation and eliminating carbonic acid gas.
Electric double layer capacitors have different forms such as button type, layered type and spiral type, and in any case are formed by alternately layering positive electrodes and negative electrodes, which are formed of polarized electrodes mainly made of carbon particles such as activated carbon, and separators which separate these two types of electrodes within an exterior case where a release valve is provided in which the inside has been impregnated with an electrolytic solution (a solution in which electrolyte is dissolved or an ionic liquid).
No chemical reactions occur at the time of charging and discharging of electric double layer capacitors, and therefore, they have advantages in which a large amount of current can be instantly charged and discharged, and the efficiency in the charging and the discharging is high. They also have other advantages in which charging and discharging is possible no less than 100,000 times, the lifetime is 10 years or more and the reliability is high. On the other hand, there is a disadvantage in that the energy density is low in comparison with lithium ion batteries and the like.
Therefore, an attempt has been made to enhance the energy density of electric double layer capacitors by optimizing the combination of the diameter of the pores of the carbon and the size of the electrolytic solution or by using Nanogate carbon or nanocarbon. Japanese Patent Application Laid-Open No. 2004-289130, for example, discloses that non-porous carbon, where multiple graphene layers are developed, is used, and thereby, the energy density can be increased close to six times higher than that of the conventional art. In addition, it is also known that the energy density can be enhanced by using nanocarbons such as carbon nanotubes.
In addition, Japanese Patent Application Laid-Open No. 2005-129924 discloses that the capacitance is increased by using alkali activated carbons having special specifications, and thus, the energy density can be enhanced.
In the case where a carbon having a high energy density is used as the material for electrodes, however, the electrodes expand at the time of charging and the electrodes contract at the time of discharging. This is because intercalation causes the volume to expand when the electrolytic solution is absorbed by the carbon in the electrodes at the time of charging, and the volume contracts when the electrolytic solution that has been absorbed by the electrodes is discharged to the outside of the electrodes at the time of discharge. In the case where Nanogate carbon or alkali activated carbon, for example, is used as the material of the electrodes, expansion of approximately 20% to 30% occurs at the time of charging, and contraction of approximately 20% to 30% occurs at the time of discharge.
In the case where an electrode expands at the time of discharge, the electrolytic solution with which a separator has been impregnated moves toward the electrode side, and the electrolytic solution with which a separator has been impregnated becomes insufficient, creating voids in the pores of a separator. As a result, a problem arises where the electric resistance of a separator becomes high.
In addition, when an electrode contracts at the time of discharge, the electrolytic solution that has been discharged from the electrode moves toward the separator side, and the electrolytic solution that cannot be contained in the separator flows over to the outside of the exterior case through a release valve. As a result, the electrolytic solution within the exterior case becomes insufficient, making the lifetime shorter, and a problem arises where the overflowed electrolytic solution causes electric short circuiting or erosion in an external circuit.
The problem of the shortage of the electrolytic solution can also arise in the case where the temperature becomes high during the operation of the electric double layer capacitor. This is caused by leaking of the electrolytic solution in liquid form from the release valve to the outside of the exterior case together with the electrolytic solution that has been decomposed and turned into a gas and carbon dioxide, which has been generated when carbon is decomposed.
Meanwhile, the expansion/contraction of the electrodes occurs only in the direction of the layering, and therefore, it is possible to reduce the expansion/contraction of the electrodes to approximately 10% by applying surface pressure to the main portions of a large cell so that the change in the amount of the electrolytic solution in the separators can be suppressed. In the case where the expansion/contraction of the electrodes is suppressed, however, the electrolytic solution does not enter into the electrodes and the area of the electric double layers is not sufficiently expanded, and thus, an increase in the capacitance stays at approximately 1.5 times in comparison with the case of conventional activated carbon. In the case where the expansion/contraction of 20% to 30% is allowed so that the electrodes are quickly filled in with a sufficient amount of electrolytic solution at the time of charging and the electrolytic solution is quickly discharged at the time of discharge, the capacitance increases to three times as much.